Commentators explain that this verse describes an ancient harvest custom, also seen in Ruth 2:4, where passersby would bless the workers, and the workers would bless them in return. The psalmist uses the absence of this joyful exchange to illustrate the complete and utter failure of the plans of God's enemies. Their efforts are so worthless, like withered grass on a roof, that no one would even think to wish them well or acknowledge their 'harvest.'